The present invention relates generally to protection against ground leakage current in electrical heating elements.
Environmental control air conditioning (A/C) systems for sensitive heat generating electronic equipment such as data processing, telecommunications, medical laboratories, industrial process control systems and network servers are used to maintain the desired temperature and humidity. Excess and insufficient humidity in rooms containing sensitive equipment can potentially harm the equipment. Excess moisture in the air can accelerate oxidation of electronic circuits, conductors and connectors and can provide high-resistance current paths that negatively affect equipment performance. Conversely, a lack of moisture increases the potential for equipment damage due to static electricity.
The ability of an A/C system to maintain both the desired temperature and humidity often requires the use of reheat systems. Frequently to remove the appropriate amount of moisture from a room, especially during the winter months, the resultant temperature from using the A/C system to control humidity is below the desired room temperature. Hence, reheat systems reheat the air being supplied to the room to maintain the desired temperature, while also assisting in the dehumidification process.
Electric heating elements typically used in air conditioning (A/C) reheat systems are metal-sheathed resistance-type heating elements composed of a metal outer sheath, resistive wire and insulation. The metal outer sheath is typically made of stainless steel material. The resistive wire can be made of a nickel-chromium material and is embedded in a magnesium oxide powder insulating material. The resistive wire is also connected to a pin connector or terminal. Power is supplied via the connector or terminal and causes the resistive wire to emit heat. The heat produced by the resistive wire is then transferred via the insulating material to the metal sheath. The air being supplied to the room is passed over the heating elements and, thus, reheated air is supplied to the room.
A variety of failures are common in metal-sheathed heating elements, such as deterioration of the metal sheath due to corrosion, moisture build-up during the summer months caused by condensation in the A/C system when the heating elements are not in use, excessive heat generated by the heating element caused by a inadequate insulation, moisture build-up in the insulation material resulting in interior corrosion of the metal sheath and bending of the heating elements, resulting in stress points and fractures. These failures frequently cause leakage of the heating element""s insulation material on surrounding surfaces as a result of cracks or holes in the metal sheath.
In addition to the contamination of surrounding surfaces caused by the failure of the heating element, the current through the resistive wire is reduced because of current leakage to ground as a result of the degraded heating element insulating material. Excess current leakage can cause short circuits and ground faults. Typical electric reheat systems utilize overcurrent protective devices, such as circuit breakers to de-energize the heating element in the event of an overcurrent condition caused by the shorting of conductors or a ground fault. While this typically protects the equipment from extensive damage due to excessively large currents, it is desirable to de-energize the heating elements prior to the excessive current caused by shorted conductors or low impedance ground faults.
The present invention is directed to detecting ground current leakage in the heating elements and to prevent failure of the heating elements from advancing beyond an initial detectable stage.
To that end, it is an object of the present invention to provide a circuit that measures the ground current due to insulation leakage in an electric heating element and de-energize the heating elements if the ground current exceeds a predetermined normal amount. All of the current carrying conductors supplying power to the heating element(s) are passed through the core of a toroidal current transformer (CT). The output of the CT is coupled to an input of a comparator device. The comparator device has a predetermined value equivalent to the threshold ground current indicative of insulation leakage in at least one of the heating elements. If the ground current measured by the CT and transmitted to the comparator exceeds the predetermined value, an SCR is fired to de-energize the coil of the contactor coil supplying power to the heating elements.
To avoid erroneous de-energization of the heating element because of switching transient current spikes, noise rejection circuitry may be added. The noise rejection circuitry includes a weighted averaging circuit that uses a second comparator to average the ground current over several cycles. This circuit requires several cycles of ground current detection above the predetermined threshold value, prior to de-energizing the heating element(s).
Because the implications of prolonged ground current leakage above a nominal level in heating elements include the possibility of severe equipment damage, such as the A/C system that includes the heating elements, the need to permanently remove power from the heating elements can be appreciated. Therefore, another aspect of the invention, includes the use a non-volatile memory device, such as a fuse, to semi-permanently de-energize the heating elements. In this circuit feature, the output of the SCR is series-connected with a fuse and the contactor coil power supply. In operation, once the ground current detection circuit detects a ground current above a predetermined threshold, the SCR is activated, which allows a current in excess of the fuse rating to be supplied to the fuse, causing the fuse to clear thereby de-energizing the contactor coil and, thus, de-energizing the heating element(s).
In yet another aspect of the present invention, it also desired to both prevent the operation of the heating element(s) when the CT is not properly connected and to allow testing of the circuit without affecting the non-volatile memory device. This is accomplished by adding a jumper connection in series with the fuse. The jumper is terminated on the jumper terminals of the CT. If the CT is not connected, no power is supplied to the heating elements. If testing of the circuit is desired, the jumper can be replaced with a fuse having a value large enough to prevent the clearing of the fuse while also allowing enough current to energize the contactor coil.
In another aspect of the present invention, light emitting diodes (LED) are used to indicate the status of the circuit and the contactor. Further, an alarm output can also be connected to the normally closed contacts of the contactor to provide an alarm output and/or interface with a display, microprocessor control system or other auxiliary device. Lastly, another aspect of the invention may include a mechanism for continuous communication of the ground current value between the CT and the display, microprocessor control system or other auxiliary device.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings.
FIG. 1 is detailed electrical schematic diagram of a control circuit according to the present invention.